<svg xmlns="http://www.w3.org/2000/svg" onload="boom();">

<script type="text/javascript">

function boom()
{
  document.getElementById("b").appendChild(document.createElementNS("http://www.w3.org/1998/Math/MathML", "math"));
  document.getElementById("defs").setAttribute("filter", "url(#a)");
}

</script>

<defs id="defs"><filter id="a"/><g id="b"><rect/></g></defs>

</svg>
